1. Field of the Invention
The present invention relates to a product-sum device, and more particularly, to a product-sum device suited for IIR (infinite impulse response) and FIR (finite impulse response) operations.
2. Description of the Related Art
A product-sum device is used as a filtering circuit in, for example, automobile and mechanical-electronic devices. The automobile and mechanical-electronic industries employ digital filters using product-sum devices and DSPs to carry out filtering processes to remove noise from detected data.
Namely, in an anti-lock braking system (ABS) for a car, for example, speed sensors are arranged near rotors which rotate with the corresponding tires. The output pulses from each sensor are counted by a pulse width counter and the output of the pulse width counter is supplied to a product-sum device. Further, an output of the product-sum device is supplied to an ABS operation unit, and an ABS actuator controls a hydraulic brake pressure, to prevent the wheels from locking, when a driver pushes hard on a brake pedal and the wheels may be locked.
Note that, the output of the pulse width counter generally contains various kinds of noise, and thus the product-sum device carries out an FIR (finite impulse response) operation to eliminate the influence of the noise included in the output of the pulse width counter.
In a conventional product-sum device, the data in each register element of a data register must be updated to carry out the FIR operation, and thus the conventional product-sum device was incapable of executing a filtering operation at high speed.
Further, in another prior art, a pointer is employed to specify data in a register element in the data register when carrying out a filtering operation. This technique, however, involves complicated operations to specify the register element into which the latest data was written and to control the pointer.
The problems of the prior art will be explained hereafter, in detail, with reference to the accompanying drawings.